Method of forming microstructures on a substrate and a microstructured assembly used for same

ABSTRACT

A method of forming microstructures on a substrate is disclosed. A microstructured assembly that may be used in the method for forming microstructures on a substrate is also disclosed. The methods and assemblies of the present disclosure can reduce the amount of air entrapped in barrier ribs formed on substrates used in Plasma Display devices.

The present disclosure generally relates to microstructured assemblies.More specifically, the present disclosure relates to methods of formingmicrostructures on a substrate that are substantially devoid of bubbles.

BACKGROUND

Advancements in display technology, including the development of plasmadisplay panels (PDPs) and plasma addressed liquid crystal (PALC)displays, have led to an interest in forming electrically-insulatingceramic barrier ribs on glass substrates. The ceramic barrier ribsseparate cells in which an inert gas can be excited by an electric fieldapplied between opposing electrodes. The gas discharge emits ultraviolet(uv) radiation within the cell. In the case of PDPs, the interior of thecell is coated with a phosphor that gives off red, green, or bluevisible light when excited by uv radiation. The size of the cellsdetermines the size of the picture elements (pixels) in the display.PDPs and PALC displays can be used, for example, as the displays forhigh definition televisions (HDTV) or other digital electronic displaydevices.

One way in which ceramic barrier ribs can be formed on glass substratesis by direct molding, which involves laminating a planar rigid mold ontoa substrate with a glass- or ceramic-forming composition disposedtherebetween. The glass- or ceramic-forming composition is thensolidified and the mold is removed. Finally, the barrier ribs are fusedor sintered by firing at a temperature of about 550° C. to about 1600°C. The glass- or ceramic-forming composition has micrometer-sizedparticles of glass frit dispersed in an organic binder. The use of anorganic binder allows barrier ribs to be solidified in a green state sothat firing fuses the glass particles in position on the substrate.However, in applications such as PDP substrates, highly precise anduniform barrier ribs with few or no defects or fractures are required.These requirements can pose challenges, especially during removal of therigid mold from the green state ribs.

PDP ribs are typically arranged in one of two pattern types. One type isreferred to as a “straight pattern.” This straight pattern is simple andcan be relatively easily manufactured on a large scale.

A flexible resin mold can be used to mold ribs having the straightpattern. The resin mold is manufactured in the following way. First, aphotosensitive resin is filled into a metal master mold having the samepattern and the same shape as those of the rib pattern to bemanufactured. Next, this photosensitive resin is covered with a plasticfilm and is cured to integrate the photosensitive resin after curingwith the film. The film is then released with the photosensitive resinfrom the metal master mold to form a flexible resin mold.

Another rib pattern type is referred to as a “lattice pattern.” Thelattice pattern can be used to improve the vertical resolution of a PDPcompared to the straight pattern, because ultraviolet rays from thedischarge display cell are better confined and are hence less likely toleak to adjacent cells. In addition, the phosphors can be applied to arelatively greater area of the discharge display cell when latticepattern ribs are employed.

Methods have previously been described that enable molding and formationof ceramic microstructures such as straight or lattice rib patterns on apatterned substrate.

For example, U.S. Pat. No. 6,247,986 B1 to Chiu et al., entitled METHODFOR PRECISE MOLDING AND ALIGNMENT OF STRUCTURES ON A 20 SUBSTRATE USINGA STRETCHABLE MOLD, and U.S. Pat. No. 7,033,534 to Chiu et al., entitledMETHOD OF FORMING MICROSTRUCTURES ON A SUBSTRATE USING A MOLD, describethe molding and aligning of ceramic barrier rib microstructures on anelectrode-patterned substrate. Such ceramic barrier rib microstructuresmay be particularly useful in electronic displays, such as PDPs and PALCdisplays, in which pixels are addressed or illuminated via plasmageneration between opposing substrates.

Although a mold can be used to manufacture ribs having the latticepattern, the removal of a rigid mold typically results in damage to theribs. A flexible mold as described herein can be applied to moldinglattice pattern ribs so that damage to the ribs may be avoided.According to existing molding technology, however, it is difficult tomanufacture a mold that eliminates the problem of rib damage upon moldremoval. In addition to problems with rib damage upon de-molding, it ispreferred not to entrap air bubbles within the mold. Large air bubblescan result in defects large enough to effectively interrupt thecontinuity of the ribs. Small air bubbles are not as disruptive, buttheir presence is not preferred.

For the lattice pattern, damage to the lateral ribs (those lyingperpendicular to the axis of removal of the flexible mold) is a problem.In addition, the rib material needs to have a sufficiently highviscosity such that it maintains the rib shape after removal of themold. However, since high viscosity material has low flowability, airbubbles in lateral grooves of the mold are difficult to eliminatecompletely.

SUMMARY

In general, the invention is directed to a method for formingmicrostructures on a substrate. The invention is further directed to amicrostructured assembly that may be used with the disclosed method.

One advantage of this disclosure is that air bubbles can be removedusing a method that employs only one application of pressure from aroller or the like in only the first direction, in contrast to atwo-step application method which would also include a secondapplication of pressure from a roller or the like traveling in thesecond direction. It is another advantage of this invention that airbubbles can be so removed using techniques that do not use vacuumdevices. For example, vacuum press molding devices limit the size of thepanels that can be processed to only at most several centimeters. Thetechniques described herein, on the other hand, can produce rib patternson large substrates.

In one aspect, the present disclosure provides a method of formingmicrostructures on a substrate. The method includes disposing a curablematerial on a substrate, where the curable material includes a viscosityof less than 12,000 cps. The method further includes contacting thecurable material with a flexible mold starting at a first end of thesubstrate and proceeding at a substantially uniform contact speed in afirst direction and applying a substantially uniform contact pressure.In addition, the method includes forming the curable material, using themold, into a lattice pattern, where the lattice pattern includes a firstset of ribs aligned in the first direction and a second set of ribsaligned in a second direction substantially orthogonal to the firstdirection, where the first set of ribs includes a pitch of less than 500μm. The method further includes curing the curable material, andremoving the mold.

In another aspect, the present disclosure provides a microstructuredassembly that includes a substrate, and a flexible mold including amicrostructured surface that opposes a surface of the substrate. Theassembly further includes a curable material disposed between thesubstrate and the microstructured surface of the flexible mold, wherethe microstructured surface of the mold is configured to impart alattice pattern into the curable material. The lattice pattern includesa first set of ribs aligned in a first direction and a second set ofribs aligned in a second direction substantially orthogonal to the firstdirection, where the first set of ribs includes a pitch of less than 500μm. The curable material includes a viscosity of less than 12,000 cps.In addition, the curable material is substantially devoid of largebubbles.

The above summary of the present invention is not intended to describeeach disclosed embodiment or every implementation of the presentinvention. The Figures and the detailed description which follow moreparticularly exemplify these embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of one embodiment of a lattice patternbarrier rib assembly.

FIGS. 2 a-e are schematic diagrams of one embodiment of a method offorming microstructures on a substrate.

FIG. 3 is a schematic diagram of a path taken by air bubbles as they areremoved from a curable material.

FIGS. 4 a-c are schematic diagrams of one embodiment of a flexible mold.

While the invention is amenable to various modifications and alternativeforms, specifics thereof have been shown by way of example in thedrawings and will be described in detail. It should be understood,however, that the intention is not to limit the invention to theparticular embodiments described. On the contrary, the intention is tocover all modifications, equivalents, and alternatives falling withinthe spirit and scope of the invention.

DETAILED DESCRIPTION

FIG. 1 is a schematic diagram of one embodiment of a lattice patternbarrier rib assembly 10. The assembly 10 includes a substrate 12 and alattice pattern 20 disposed on a major surface 14 of the substrate 12.The lattice pattern 20 includes a first set of ribs 22 aligned in afirst direction 16 and a second set of ribs 24 aligned in a seconddirection 18. The first direction 16 and the second direction 18 aresubstantially orthogonal.

In general, plasma display panels (PDPs) can include various substrateelements. The back substrate assembly (e.g., assembly 10), which can beoriented away from the viewer, can include a back substrate (e.g.,substrate 12) with independently addressable parallel electrodes (notshown in FIG. 1) formed on or in a major surface of the back substrate.The back substrate can be formed from a variety of compositions, e.g.,glass. Microstructures (e.g., lattice pattern 20) are formed on a majorsurface of the back substrate and include barrier rib portions that arepositioned between electrodes and separate areas in which red (R), green(G), and blue (B) phosphors are deposited. PDPs can also include a frontsubstrate assembly that includes a glass substrate and a set ofindependently addressable parallel electrodes. These front electrodes,also called sustain electrodes, are oriented orthogonally to the backelectrodes, also referred to as address electrodes.

In a completed display, the area between the front and back substrateassemblies can be filled with an inert gas. To light up a pixel, anelectric field is applied between crossed sustain and address electrodeswith enough strength to excite the inert gas atoms therebetween. Theexcited inert gas atoms emit uv radiation, which causes the phosphor toemit red, green, or blue visible light.

It may be preferred that the back substrate is a transparent glasssubstrate. Typically, for PDP applications, the back substrate is madeof soda lime glass that is optionally substantially free of alkalimetals. The temperatures reached during processing can cause migrationof the electrode material in the presence of alkali metal in thesubstrate. This migration can result in conductive pathways betweenelectrodes, thereby shorting out adjacent electrodes or causingundesirable electrical interference between electrodes known as“crosstalk.” The front substrate is typically a transparent glasssubstrate that can have the same or about the same coefficient ofthermal expansion as that of the back substrate.

Electrodes are strips of conductive material. The electrodes are formedof a conductive material, e.g., copper, aluminum, or a silver-containingconductive frit. The electrodes can also be a transparent conductivematerial, such as indium tin oxide, especially in cases where it isdesirable to have a transparent display panel. The electrodes arepatterned on the back substrate and front substrate. For example, theelectrodes can be formed as parallel strips spaced about 120 μm to 360μm apart, having widths of about 50 μm to 75 μm, thicknesses of about 2μm to 15 μm, and lengths that span the entire active display area thatcan range from a few centimeters to several tens of centimeters. In someinstances, the widths of the electrodes can be narrower than 50 μm orwider than 75 μm, depending on the architecture of the microstructures.

In some embodiments, barrier ribs portions in PDPs typically haveheights of about 120 μm to 140 μm and widths of about 20 μm to 75 μm. Itmay be preferred that the pitch (number per unit length) of the barrierribs matches the pitch of the electrodes. In other embodiments, thepitch of the barrier ribs in the mold can be larger or smaller than thepitch of the electrodes, and the mold can be stretched to align the ribswith the electrodes, e.g., as described in U.S. Pat. No. 6,247,986 B1 toChiu et al., entitled METHOD FOR PRECISE MOLDING AND ALIGNMENT OFSTRUCTURES ON A SUBSTRATE USING A STRETCHABLE MOLD.

When using the techniques described herein to form microstructures on asubstrate (such as barrier ribs for a PDP), the curable material fromwhich the microstructures are formed can be a slurry or paste, e.g., asdescribed in U.S. Pat. No. 6,352,763 B1 to Dillon et al., entitledCURABLE SLURRY FOR FORMING CERAMIC MICROSTRUCTURES ON A SUBSTRATE USINGA MOLD. In an illustrative aspect, the techniques as described hereinmay include using a slurry that contains a ceramic powder, a curableorganic binder, and a diluent, e.g., the slurries described in U.S. Pat.No. 6,352,763 B1. When the binder is in its initial uncured state, theslurry can be shaped and aligned on a substrate using a mold. Aftercuring the binder, the slurry is in at least a semi-rigid state that canretain the shape in which it was molded. This cured, rigid state isreferred to as the green state, just as shaped ceramic materials arecalled “green” before they are sintered. When the slurry is cured, themold can be removed from the green state microstructures. The greenstate material can subsequently be debinded and/or fired. Debinding, orburn out, occurs when the green state material is heated to atemperature at which the binder can diffuse to a surface of the materialand volatilize. Debinding is usually followed by increasing thetemperature to a predetermined firing temperature to sinter or fuse theparticles of the ceramic powder. After firing, the material can bereferred to as fired material. Fired microstructures are referred toherein as ceramic micro structures.

Generally, the techniques described herein typically use a mold to formthe microstructures. The mold may be a flexible polymer sheet having asmooth surface and an opposing microstructured surface. The mold can bemade by compression molding of a thermoplastic material using a mastertool that has a microstructured pattern. In some embodiments, the moldcan also be made of a curable material that is cast and cured onto athin, flexible polymer film. The microstructured mold can be formed, forexample, using techniques disclosed in U.S. Pat. No. 5,175,030 to Lu etal., entitled MICROSTRUCTURE-BEARING COMPOSITE PLASTIC ARTICLES ANDMETHOD OF MAKING; U.S. Pat. No. 5,183,597 to Lu, entitled METHOD OFMOLDING MICROSTRUCTURE BEARING COMPOSITE PLASTIC ARTICLES; and U.S. Pat.No. 7,033,534 to Chiu et al., entitled METHOD FOR FORMINGMICROSTRUCTURES ON A SUBSTRATE USING A MOLD.

FIGS. 2 a-e are schematic diagrams of one embodiment of a method offorming microstructures on a substrate. In FIG. 2 a, an apparatus 100for molding microstructures on a substrate is illustrated. The apparatus100 includes a substrate 110, a flexible mold 130, and a laminatingroller 140. The substrate 110 can be any substrate described herein. Theflexible mold 130 includes a flexible backing 132 and a microstructuredsurface 134 on a major surface of the flexible backing 132. Themicrostructured surface 134 includes rib forming regions 136 and landforming regions 138. The flexible mold 130 in this embodiment isconfigured and arrayed to form barrier regions (e.g., barrier ribs 124of FIG. 2 e) on substrate 110.

Generally, a roller 140 or other pressure application device can beprovided to provide pressure to the mold 130 and a curable material(e.g., curable material 120 of FIG. 2 b) to drive a portion of thecurable material into rib forming regions 136 within the microstructuredsurface 134 of the mold 130.

As shown in FIG. 2 b, a curable material 120 is disposed on a majorsurface 112 of substrate 110. Typically, the curable material 120 iscoated on the substrate 110 using a coating technique that can producesubstantially uniform coatings, e.g., knife coating, screen printing,extrusion coating, and reverse gravure coating. The curable material 120may include any suitable material or materials as described herein.

The curable material 120 can be coated on one or more regions of thesubstrate 110. In some embodiments, the curable material 120 can bedisposed on substantially the entire major surface 112 of substrate 110.In some embodiments, the curable material 120 may be disposed on region116 of the substrate. Edge portions 114 of major surface 112 can be leftsubstantially free from curable material 120 to provide areas forhandling the substrate or, particularly in the case of PDP and otherdisplay technologies, areas free of curable material where sealing tothe front panel is performed and electrical connections can be made withelectrodes patterned on the substrate (not shown).

Generally, the thickness of the curable material 120 varies by no morethan 10%. It may be preferred that the thickness of the curable material120 varies by no more than 5%. It may be more preferred that thethickness of curable material 120 varies by no more than 2%. In oneembodiment, the curable material 120 has an average thickness of about75 μm. In another embodiment, the average thickness of curable material120 may be about 50 μm.

It may be preferred that the area of the substrate 110 upon which alattice pattern is desired (e.g., region 116) has been predeterminedprecisely beforehand, and the curable material 120 is disposed only uponthe area. The area of the substrate 110 having no curable material 120disposed thereon (e.g., edge portions 114) can be used for handlingduring processing, and for electrical connections in the case that theassembly is to be used in a PDP.

In FIGS. 2 b-c, the mold 130 contacts the curable material 120 beginningat a first end 118 of the substrate 110 as pressure is applied to themold 130 along direction 150. The roller 140 may be used to applypressure to the mold 130 such that the mold 130 contacts the curablematerial 120 beginning at the first end 118 of the substrate 110. Themold 130 may be made to contact the curable material 120 at any suitablecontact speed in direction 150. It may be preferred that the mold 130contact curable material 120 at a substantially uniform contact speed.Further, any suitable contact pressure may be applied to mold 130 suchthat it contacts curable material. It may be preferred that asubstantially uniform contract pressure is applied to the mold 130. Thecurable material 120 is deformed such that the rib forming regions 136of the microstructured surface 134 of the flexible mold 130 becomefilled. It may be preferred that the contact speed and contact pressureare chosen such that the curable material 120 is not entirely squeezedout from under the microstructured surface 134 of the flexible mold 130,thus leaving land regions in the curable material 120 corresponding toland forming regions 138 (e.g., land regions 126 of FIG. 2 e).

As the mold 130 contacts the curable material 120, the curable materialis formed into a lattice pattern (e.g., lattice pattern 20 of FIG. 1).For example, FIG. 2 d illustrates one embodiment of a microstructuredassembly 160. The microstructured assembly 160 includes the substrate110, the flexible mold 130, and the curable material 120. Themicrostructured surface 134 of mold 130 is configured to impart alattice pattern into the curable material 120. In some embodiments, thelattice pattern includes a first set of ribs (e.g., first set of ribs 22of FIG. 1) aligned in a first direction (e.g., direction 16 of FIG. 1)and a second set of ribs (e.g., second set of ribs 24 of FIG. 1) alignedin a second direction (e.g., second direction 18 of FIG. 1). Further,the lattice pattern can include land regions 126. As illustrated in FIG.2 d, ribs 124 are included in the second set of ribs, whereas the firstset of ribs are not shown.

In FIG. 2 d, the curable material 120 is cured to form ribs 124 on majorsurface 112 of substrate 110. Curing of the material 120 can take placein a variety of ways depending on the binder used. For example, thematerial can be cured using one or more curing devices providing visiblelight, ultraviolet light, e-beam radiation, or other forms of radiation,or by heat curing or by cooling to solidification from a melted state.For radiation curing, radiation can be propagated through the substrate110, through the mold 130, or through the substrate 110 and the mold130. Preferably, the cure system chosen facilitates adhesion of thecured material 120 to the substrate 110.

After curing the material 120, the mold 130 can be removed (e.g., bywinding the mold onto a receiving element, e.g., a roller). A flexiblemold can aid in mold removal because the mold can be peeled back so thatthe demolding force can be focused on a smaller surface area. It may bepreferred that a mold release material is included either as a coatingon the patterned surface of the mold or in the material that is hardenedto form the lattice pattern itself A mold release material becomes moreimportant as higher aspect ratio structures are formed. Higher aspectratio structures make demolding more difficult and can lead to damage tothe microstructures.

After the mold 130 is removed, what remains is the substrate 110 havinga plurality of hardened microstructures adhered thereon. Depending onthe application, this can be the finished product. In otherapplications, such as substrates that will have a plurality ofmicrostructures, the hardened material contains a binder that ispreferably removed by debinding at elevated temperatures. Afterdebinding, or burning out of the binder, firing of the green stateceramic microstructures is performed to fuse the glass particles orsinter the ceramic particles in the material of the microstructures.This increases the strength and rigidity of the microstructures.Shrinkage can also occur during firing as the microstructure densifies.Fired microstructures maintain their positions and their pitch accordingto the substrate pattern.

For PDP display applications, phosphor material is applied between thebarrier regions of the microstructures. The substrate then can beinstalled into a display assembly. This involves aligning a frontsubstrate having sustain electrodes with the back substrate havingaddress electrodes, microstructures, and phosphor such that the sustainelectrodes are perpendicular with the address electrodes. The areasthrough which the opposing electrodes cross define the pixels of thedisplay. The space between the substrates is then evacuated and filledwith an inert gas as the substrates are bonded together and sealed attheir edges.

It will be recognized that other articles can also be formed using asubstrate with the molded microstructures. For example, the moldedmicrostructures can be used to form capillary channels for applicationssuch as electrophoresis plates. In addition, the molded microstructurescould be used for plasma displays or other applications that producelight.

As the mold contacts the curable material, air may become trappedbetween the microstructured surface of the mold and the curablematerial. This trapped air may in turn form air bubbles within themicrostructures formed in the curable material. It may be preferred thatany trapped air be removed from between the mold and the curablematerial.

In the present application, “small bubbles” refers to air bubbles thatare less that half the rib height (or other microstructural featuresize) in size. The presence of such small bubbles is not preferred, butmay not disrupt the continuity of the ribs or other microstructuralfeatures, and hence, may not significantly degrade functionality. “Largebubbles” refers to air bubbles which are about half the rib height orlarger in size. Large bubbles can disrupt the continuity of the ribs orother microstructural features and significantly degrade functionality.In the present application, the word “defects” refers to damaged ribs orstructures, such as broken ribs or ribs with missing sections, as wellas to large bubbles.

One way in which the trapped air can be removed is through grooves that,in some embodiments, form the microstructured surface of the mold. Forexample, FIG. 3 is a schematic diagram of a path taken by an air bubbleas it is removed during the application of a flexible mold having amicrostructured surface to a curable material. In FIG. 3, the flexiblemold (not shown) is applied in a first direction 212. The latticepattern 220 that is formed in the curable material 216 includes a firstset of ribs 222 aligned in the first direction 212. Lattice pattern 220further includes a second set of ribs 224 aligned in a second direction214. A first air bubble 230 is shown schematically within one rib 226 ofthe second set of ribs 224. For the first air bubble 230 to escapeduring application of the flexible mold, it must migrate into an areabetween a rib of the first set of ribs 222 and the mold, so that it canbe squeezed out of the curable material 216 along the direction ofapplication of the flexible mold, i.e., the first direction 212. Abubble that has so migrated is shown schematically as second air bubble232.

One technique that may aid in the removal of trapped air may includecontrolling certain dimensions of the rib forming regions (i.e.,grooves) of a microstructured surface of a mold.

FIGS. 4 a-c are schematic diagrams of a flexible mold 300. The flexiblemold 300 is applied to a curable material in direction 310 as furtherdescribed herein. The flexible mold 300, which includes a negative imageof the lattice pattern to be formed in the curable material, will haverib forming regions where the rib assembly is to have ribs. The mold 300includes a first set of rib forming regions 320 and a second set of ribforming regions 330. It is to be understood that the rib forming regions320 and 330 of the mold 300 will form ribs in a curable material thathave substantially the same shape and dimensions as the correspondingrib forming regions. Note that the first set of rib forming regions 320are aligned in the first direction 310 and the second set of rib formingregions 330 are aligned in the second direction 312. In someembodiments, the first set of rib forming regions 320 need not beidentical in shape and size to the second set of rib forming regions330.

As shown in FIG. 4 b, each rib forming region of the first set of ribforming regions 320 includes an opening width 322 and a bottom width324. Further, as shown in FIG. 4 c, each rib forming region of thesecond set of rib forming regions 330 includes an opening width of 332and a bottom width of 334. In other embodiments, the rib forming regionsmay have opening widths equal in size to the bottom widths.Alternatively, the opening width may be greater than the bottom widthfor one or more rib forming regions of one of the first set of ribforming regions 320 and second set of rib forming regions 330 or bothsets of rib forming regions. Further, the side walls of the rib formingregions may be any suitable shape, e.g., curved, straight, parabolic.The side walls of each rib forming region may also include textured orpatterned surfaces.

Each rib forming region of the first set of rib forming regions 320 hasa depth 328. Similarly, each rib forming region of the second set of ribforming regions 330 has a depth 338. The depths of each rib formingregion may be the same for the first set of rib forming regions 320 orthe second set of rib forming regions 330. Alternatively, the depth ofeach rib forming region of the first set of rib forming regions 320 orthe second set of rib forming regions 330 may vary.

Further, each rib forming region of the first set of rib forming regions320 may have the same shape and dimensions as the rest of the ribforming regions in the first set; alternatively, the rib forming regionsof the first set of rib forming regions 320 may have different shapesand dimensions. In other embodiments, the second set of rib formingregions 330 may include rib forming regions that have the same shapesand dimensions, or the rib forming regions may have varying shapes anddimensions.

Each rib forming region of the first set of rib forming regions 320includes an average width that is one-half the sum of the opening width322 and the bottom width 324. Similarly, the average width of each ribforming region of the second set of rib forming regions 330 is one-halfthe sum of the opening width 332 and the bottom width 334. The averagewidth of each rib forming region of the first set of rib forming regions320 and the average width of each rib forming region of the second setof rib forming regions 320 need not be equal.

The first set of rib forming regions 320 includes a pitch 326, and thesecond set of rib forming regions 330 includes a pitch 336. The pitch326 of the first set of rib forming regions 320 and the pitch 336 of thesecond set of rib forming regions 330 may be equal. In some embodiments,the pitch 326 of the first set of rib forming regions 320 may be greateror less than the pitch 336 of the second set of rib forming regions 330.

Several factors may influence the removal of air bubbles from thecurable material. For example, the viscosity of the curable material,the pitch 326 of the first set of rib forming regions 320, and the pitch336 of the second set of rib forming regions 330 may affect the removalof air bubbles. Other parameters may also have an effect. For example,the ratio of the average width of each rib forming region of the secondset of rib forming regions 330 and the average width of each rib formingregion of the first set of rib forming regions 320, the shape of the ribforming regions, and the coated thickness of the curable material mayinfluence bubble formation and removal. Another such parameter is theapplication (roller) loading or pressure as the flexible mold is beingapplied to the curable material and the speed or rate of the application(roller travel).

To aid in preventing bubble formation, it may be preferred that theviscosity of the curable material is less than 12,000 cps. Further, itmay be preferred that the pitch of the first set of rib forming regions320 is less than 500 μm. It may be more preferred that the pitch of thefirst set of rib forming regions is less than 300 μm.

Further, it may be preferred that the ratio of the average width of eachrib forming region of the second set of rib forming regions 330 and theaverage width of each rib forming region of the first set of rib formingregions is at least 1.5. Without wishing to be bound by any theory, itis believed that widening each rib forming region of the second set ofrib forming regions 330 with respect to the width of each rib formingregion of the first set of rib forming regions 320 alters the pressuredrops in the respective channels during application of the flexible moldin such a way as to enable ever-smaller bubbles to escape by the routeshown schematically in FIG. 3. One skilled in the art will appreciatethat increasing the value of the ratio of the average width of each ribforming region of the second set of rib forming regions and the averagewidth of each rib forming region of the first set of rib forming regionsbeyond 1.5 will lead progressively to the elimination of smaller andsmaller air bubbles, if desired.

Also, the length of the path an air bubble must traverse in order toescape by the route shown schematically in FIG. 3 may further influencethe removal of air bubbles from the curable material. For example, theedge-to-edge bottom distance of the first set of rib forming regions 320may in some instances be less than 150 μm or more than 300 μm. Oneskilled in the art will appreciate that if this distance is less than150 μm, the value of the ratio of the average width of each rib formingregion of the second set of rib forming regions 330 and the averagewidth of each rib forming region of the first set of rib forming regions320 effective for bubble removal may be lower than 1.5. Conversely, ifthe distance is greater than 300 μm, the value of this ratio necessaryfor effective bubble removal may be greater than 1.5.

Another factor that may influence bubble removal is the quantity ofcurable material disposed on the substrate prior to the flexible moldcontacting the curable material. As further described herein, thecurable material is disposed on the substrate in an area of thesubstrate upon which the lattice rib pattern is intended to be formed(e.g., region 116 of substrate 110 as illustrated in FIG. 2 b).Conditions may be selected such that the amount of curable materialsqueezed out from under the microstructured surface of the flexible moldis substantially equal to the amount of curable material squeezed upinto the rib forming regions of the microstructured surface. The firstset of rib forming regions, which correspond to the first set of ribsaligned in the first direction, provide an air channel by which airbubbles can escape.

If, however, the amount of curable material squeezed out from under themicrostructured surface of the flexible mold is substantially in excessof the amount of curable material squeezed up into the rib formingregions, a bank of curable material may be formed ahead of the advanceof the flexible mold. This results in a “paste overflow” condition. Whenthe bank is created, one or more rib forming regions of the second setof rib forming regions (e.g., second set of rib forming regions 330 ofFIG. 4 a) become filled out of sequence. The first set of rib formingregions provides an air channel by which air bubbles can escape (see,e.g., FIG. 3). However, when one or more rib forming regions of thesecond set of rib forming regions become filled out of sequence, thisair channel provided by the first set of rib forming regions becomesblocked; therefore, some of the air bubbles may not completely escape.

Not only may the amount of curable material disposed on the substrateaffect air bubble removal, the viscosity of the curable material alongwith the pressure or loading applied by the roller, and the speed atwhich the roller travels may also affect air bubble removal. Forexample, too low a viscosity for the curable material can also lead topaste overflow.

EXAMPLES Example 1

A metal mold was prepared to the desired dimensions of the latticepattern assembly to be made. The metal mold includes a microstructuredsurface having a first set of rib forming regions aligned in a firstdirection and a second set of rib forming regions aligned in a seconddirection substantially orthogonal to the first direction. The first setof rib forming regions had a pitch of 300 μm. Each rib forming region ofthe first set of rib forming regions had a height of 208 μm, an openingwidth of 55 μm, and a bottom width of 115 μm. The dimension of these ribforming regions would form a rib having a taper angle of 82 degrees. Thetaper angle is the included angle at the base of a rib. A rib formingregion with equal opening and bottom widths would form a rib having ataper angle of 90 degrees. The second set of rib forming regions had apitch of 500 μm. Each rib forming region had a height of 208 μm, anopening width of 37 μm, and a bottom width of 160 μm, which would resultin a rib taper angle of 75 degrees.

A mixture of 99% by wt. of an aliphatic urethane acrylate oligomer(Photomer 6010™, manufactured by Henkel Co.) and 1% by wt.2-hydroxyl-2-methyl-1-phenyl-propane-1-one (Darocure 1173™, manufacturedby Ciba-Gigy) as a photoinitiator was prepared. An amount slightly inexcess of that needed to completely fill the microstructured surface ofthe mold was placed between a PET film and the metal mold. The mixturewas cured by exposure to radiation of wavelength 300-400 nm for 30 sec.The thus-cured urethane acrylate polymer adhered strongly to the PETfilm and was released together with the PET film from the metal mold toobtain a flexible and transparent plastic mold. The rib forming regionsin the flexible mold had the same shape and the same dimensions as therib forming regions in the metal mold.

A ceramic paste was prepared to serve in the molding method as thecurable material. 21.0 g of dimethacrylate of bisphenol A diglycidylether (Kyoeisha Chemical Co., Ltd.), 9.0 g of triethylene glycoldimethacrylate (Wako Pure Chemical Industries, Ltd.), 30.0 g of1,3-butandiol (Wako Pure Chemical Industries, Ltd.) as a dilutant, 0.3 gof bis(2,4,6-trimethylbenzoyl)-phenylphospheneoxide (Irgacure 819, madeby Ciba-Geigy) as an initiator, 3.0 g of phosphated polyoxyalkyl polyol(POCA) as a surfactant, and 180.0 g of a mixture of glass frit andceramic particles (RFW-030, made by Asahi Glass Co) were mixed to obtainthe photocurable ceramic paste. The paste viscosity was 6000 cps (asmeasured at 22° C. and 20 rpm with spindle No. 5 on a type Bviscometer).

The ceramic paste was coated onto a glass substrate to a thickness of200 μm, and then the flexible mold was applied in a first direction,with a roller, onto the paste. Afterwards, the assembly was exposed toradiation of wavelength 400-500 nm for 30 s to cure the paste. Theflexible mold was peeled from the substrate in the first direction. Thesubstrate and cured ribs assembly was then sintered at 550° C. for 1 hto burn out the organic part of the ribs. After the sintering, the ribswere evaluated using an optical microscope. Either damage to a rib or abubble in a rib that was so large as to significantly disrupt thecontinuity of the rib were regarded as defects. Sometimes, very smallair bubbles are observed on the very tops of the lateral ribs. Thesesmall air bubbles are approximately an order of magnitude smaller thanthe heights of the ribs, so they do not significantly disrupt thecontinuity of the rib. No defects were observed in this specimen. Smallair bubbles were observed in this specimen.

Defect level in this and other Examples was defined as a ratio of thenumber of defects detected to the number of rib segments of the set ofsecond direction ribs in the visual field (7.5 mm in diameter) of themicroscope. This measurement was done in seven randomly-selected areason the specimen, and the average of the seven results is reported. Thedefect level of Example 1 was 0.0%.

Examples 2 and 3

The flexible molds were made as described in Example 1. The viscosity ofthe paste was varied by varying the solids content (glass frit andceramic particles). Solids content was 90.0 g in Example 2 and 145.0 gin Example 3. All other components were identical in type and loadinglevel as those in Example 1. The paste viscosities were 1800 cps forExample 2 and 4800 cps for Example 3.

Lattice pattern rib assemblies were made in the same way as inExample 1. The defect level was measured by microscopy. The defectlevels of both Examples 2 and 3 were 0.0%. Small bubbles were observedin these specimens.

Comparative Examples 1 and 2

The flexible molds were made as described in Example 1. The viscosity ofthe paste was varied by varying the solids content (glass frit andceramic particles). Solids content was 220.0 g in Comparative Example 1and 270.0 g in Comparative Example 2. All other components wereidentical in type and loading level as in Example 1. The pasteviscosities were 12,600 cps for Comparative Example 1 and 27,300 cps forComparative Example 2.

Lattice pattern rib assemblies were made in the same way as inExample 1. The defect level was measured by microscopy. The defectlevels were 0.1% for Comparative Example 1 and 3.3% for ComparativeExample 2. Small bubbles were also observed in these specimens.

Comparative Example 3

A flexible mold and a ceramic paste were made as described in Example 1,with the exception that the first direction and second direction of themold were reversed. Thus, the pitch in the first direction was 500 μm.The paste viscosity was 6000 cps. Lattice pattern rib assemblies weremade in the same way as in Example 1.

The defect level was measured by microscopy. Many defects were observedin this specimen. All cross members included defects, which means thatthe defect level is 100% in Comparative Example 3. Small bubbles werealso observed in this specimen.

Example 4

A flexible plastic mold having lattice pattern microstructured surfacewas prepared using the same materials as in Example 1.

The microstructured surface in the mold corresponded to ribs having thefollowing dimensions. Ribs of the first set of ribs had a pitch of 300μm, a height of 200 μm, an opening width of 50 μm, and a bottom width of100 μm. The ribs of the second set of ribs had a pitch of 500 μm, aheight of 200 μm, an opening width of 150 μm, and a bottom width of 220μm.

The average width of each rib of the first set of ribs was thus(50+100)/2=75 and, the average width of each rib of the second set ofribs was thus (150+220)/2=185. The ratio of the average width of eachrib of the second set of ribs and the average width of each rib of thefirst set of ribs was thus 185/75, or about 2.5.

A ceramic paste was prepared to serve in the molding method as thecurable material. 21.0 g of dimethacrylate of bisphenol A diglycidylether (Kyoeisha Chemical Co., Ltd.), 9.0 g of triethylene glycoldimethacrylate (Wako Pure Chemical Industries, Ltd.), 30.0 g of1,3-butandiol (Wako Pure Chemical Industries, Ltd.) as a dilutant, 0.2 gof bis(2,4,6-trimethylbenzoyl)-phenylphospheneoxide (Irgacure 819, madeby Ciba-Geigy) as an initiator, 1.5 g of phosphateed polyoxyalkyl polyol(POCA) and 1.5 g of sodium dodecylbenzenesulfonate (NeoPelex #25, madeby Kao Co.) as surfactants, and 270.0 g of a mixture of glass frit andceramic particles (RFW-030, made by Asahi Glass Co) were mixed to obtainthe photocurable ceramic paste. The paste viscosity was 7300 cps (asmeasured at 22° C. and 20 rpm with spindle No. 5 on a type Bviscometer).

The ceramic paste was coated on a glass substrate to a thickness of 130μm by a blade coater, and then the flexible mold was applied along thefirst direction onto the paste using a rubber roller.

Afterwards, the assembly was exposed to radiation of wavelength 400-500nm for 30 s to cure the paste. The flexible mold was peeled from thesubstrate in the first direction.

The sizes of air bubbles near the tops of the ribs of the second set ofribs were measured at 18 points by microscopy. The average air bubblesize is summarized in Table 1. No defects or small air bubbles wereobserved in Example 4.

Examples 5 and 6

Flexible plastic molds having different rib forming region shapes fromExample 4 were prepared.

The rib shapes corresponding to those rib forming region shapes aredescribed as follows.

Example 5

Ribs of the first set of ribs had a pitch of 300 μm, a height of 200 μm,an opening width of 50 μm, and a bottom width of 100 μm. The ribs of thesecond set of ribs had a pitch of 500 μm, a height of 200 μm, an openingwidth of 125 μm, and a bottom width of 190 μm.

The average width of each rib of the first set of ribs was thus(50+100)/2=75 and, the average width of each rib of the second set ofribs was thus (125+190)/2=157.5. The ratio of the average width of eachrib of the second set of ribs and the average width of each rib of thefirst set of ribs was thus 157.5/75=2.1.

Example 6

Ribs of the first set of ribs had a pitch of 300 μm, a height of 200 μm,an opening width of 50 μm, and a bottom width of 100 μm. The ribs of thesecond set of ribs had a pitch of 500 μm, a height of 200 μm, an openingwidth of 100 μm, and a bottom width of 170 μm.

The average width of each rib of the first set of ribs was thus(50+100)/2=75 and, the average width of each rib of the second set ofribs was thus (100+170)/2=135 The ratio of the average width of each ribof the second set of ribs and the average width of each rib of the firstset of ribs is thus 135/75=1.8.

The lattice pattern ribs were formed by using the mold as described inExample 4. The sizes of air bubble near the tops of the ribs of thesecond set of ribs were measured at 18 points by microscopy. The averageair bubble size is summarized in Table 1. No defects or small airbubbles were observed in Example 5 or 6.

Examples 7 and 8

Flexible plastic molds that have different rib forming region shapesfrom Example 4 were prepared.

The rib shapes corresponding to those rib forming region shapes aredescribed as follows.

Example 7

Ribs of the first set of ribs had a pitch of 300 μm, a height of 200 μm,an opening width of 50 μm, and a bottom width of 100 μm. The ribs of thesecond set of ribs had a pitch of 500 μm, a height of 200 μm, an openingwidth of 75 μm, and a bottom width of 140 μm.

The average width of each rib of the first set of ribs was thus(50+100)/2=75 and, the average width of each rib of the second set ofribs was thus (75+140)/2=107.5. The ratio of the average width of eachrib of the second set of ribs and the average width of each rib of thefirst set of ribs was thus 107.5/75=1.4.

Example 8

Ribs of the first set of ribs had a pitch of 300 μm, a height of 200 μm,an opening width of 60 μm, and a bottom width of 120 μm. The ribs of thesecond set of ribs had a pitch of 500 μm, a height of 200 μm, an openingwidth of 60 μm, and a bottom width of 110 μm.

The average width of each rib of the first set of ribs is thus(60+120)/2=90 and, the average width of each rib of the second set ofribs is thus (60+110)/2=85. The ratio of the average width of each ribof the second set of ribs and the average width of each rib of the firstset of ribs is thus 85/90=0.94.

The lattice pattern ribs were formed by using the mold as described inExample 4. The sizes of air bubbles near the tops of the ribs of thesecond set of ribs were measured at 18 points by microscopy. The averageair bubble size is summarized in Table 1. The average sizes of airbubbles were 18 μm and 25 μm in examples 7 and 8, respectively. Nodefects were observed in these specimens, however.

TABLE 1 Ratio Air bubble size Example 4 2.5 0 micron Example 5 2.1 0micron Example 6 1.8 0 micron Example 7 1.4 18 micron  Example 8 0.9 25micron 

Example 9

A metal mold was prepared to the desired dimensions of the latticepattern assembly to be made. The metal mold includes a microstructuredsurface having a first set of rib forming regions aligned in a firstdirection and a second set of rib forming regions aligned in a seconddirection substantially orthogonal to the first direction. The first setof rib forming regions had a pitch of 300 μm. Each rib forming region ofthe first set of rib forming regions had a height of 200 μm, an openingwidth of 60 μm, and a bottom width of 120 μm. The second set of ribforming regions had a pitch of 500 μm, a height of 200 μm, an openingwidth of 40 μm, and a bottom width of 160 μm, resulting in a rib taperangle of 75 degrees.

A mixture of 99% by wt. of an aliphatic urethane acrylate oligomer(Photomer 6010™, manufactured by Henkel Co.) and 1% by wt.2-hydroxyl-2-methyl-1-phenyl-propane-1-one (Darocure 1173™, manufacturedby Ciba-Gigy) as a photoinitiator was prepared. An amount slightly inexcess of that needed to completely fill the microstructure of the moldwas placed between a PET film and the metal mold. The mixture was curedby exposure to radiation of wavelength 300-400 nm for 30 sec. Thethus-cured urethane acrylate polymer adheres strongly to the PET film,and was released together with the PET film from the metal mold toobtain a flexible and transparent plastic mold. The grooves in theflexible mold had the same shape and the same dimensions as the ribs inthe metal mold.

A ceramic paste was prepared to serve in the molding method as thecurable material. 21.0 g of dimethacrylate of bisphenol A diglycidylether (Kyoeisha Chemical Co., Ltd.), 9.0 g of triethylene glycoldimethacrylate (Wako Pure Chemical Industries, Ltd.), 30.0 g of1,3-butandiol (Wako Pure Chemical Industries, Ltd.) as a dilutant, 0.2 gof bis(2,4,6-trimethylbenzoyl)-phenylphospheneoxide (Irgacure 819, madeby Ciba-Geigy) as an initiator, 1.5 g of phosphateed polyoxyalkyl polyol(POCA) and 1.5 g of sodium dodecylbenzenesulfonate (NeoPelex #25, madeby Kao Co.) as surfactants, and 270.0 g of a mixture of glass frit andceramic particles (RFW-030, made by Asahi Glass Co) were mixed to obtainthe photocurable ceramic paste. The paste viscosity was 7300 cps (asmeasured at 22° C. and 20 rpm with spindle No. 5 on a type Bviscometer).

The ceramic paste was coated on a glass substrate to a thickness of 110μm by a blade coater. The coating area was a 950×540 mm rectangle thatcorresponded to the lattice pattern area of the mold. Then the flexiblemold was applied along the first direction onto the 110 micron thicklayer of paste by using a 30 kg, 200 mm diameter roller at a rate of 42mm/s. Since no additional loading was given to the mold, the totalloading to the mold is 30 kg/950 mm, or about 0.032 kg/mm. Afterwards,the assembly was exposed to radiation of wavelength 400-500 nm for 30 sto cure the paste. The flexible mold was peeled from the substrate inthe first direction.

The amount of paste overflow resulting from the application step wasobtained by measuring the difference between the paste coating areabefore the application of the flexible mold and the paste coating areaafter the application of the flexible mold. The specimen of Example 9showed no difference in paste coating area before and after theapplication of the flexible mold, which indicates that the conditions ofExample 9.

After the removal of the mold, the assembly of substrate and latticepattern ribs was sintered at 550° C. for 1 h to burn out the organicpart of the ribs.

After the sintering, rib defects were measured by optical microscopy. Nodefects were observed in the entire area (950×540 mm) of the specimen ofExample 9. Small air bubbles were observed in this specimen.

Examples 10 and 11

The flexible plastic molds and photocurable ceramic paste were made asdescribed in Example 9.

The ceramic paste was coated on a glass substrate to a thickness of 110μm by a blade coater. The coating area was a 950×540 mm rectangle thatcorresponded to the lattice pattern area of the mold. Then the flexiblemold was applied along the first direction onto the 110 micron thicklayer of paste. For Example 10, a 30 kg, 200 mm diameter roller was usedat a rate of 20 mm/s. For Example 11, a 100 kg, 200 mm diameter rollerwas used at a rate of 42 mm/s. Since no additional loading was given tothe mold, the total loading to the mold is 30 kg/950 mm, or about 0.032kg/mm for Example 10, and is 100 kg/950 mm, or about 0.105 kg/mm forExample 11. Afterwards, the assembly was exposed to radiation ofwavelength 400-500 nm for 30 s to cure the paste. The flexible mold waspeeled from the substrate in the first direction.

The amount of paste overflow resulting from the application step wasobtained by measuring the difference between the paste coating areabefore the application of the flexible mold and the paste coating areaafter the application of the flexible mold. The specimens of Examples 10and 11 showed no difference in paste coating area before and after theapplication of the flexible mold, which indicates that the conditions ofExamples 10 and 11 did not lead to “paste overflow” conditions.

After the removal of the mold, the assembly of substrate and latticepattern ribs was sintered at 550° C. for 1 h to burn out the organicpart of the ribs.

After the sintering, rib defects were measured by optical microscopy. Nodefects were observed in the entire area (950×540 mm) of the specimensof Examples 10 and 11. Small air bubbles were observed in thesespecimens.

Comparative Example 4

The flexible plastic molds were made as described in Example 9. Thepaste viscosity was lowered by decreasing the content of the RFW-030 inthe paste. 180.0 g of RFW-030 was used instead of the 270.0 g used inExample 9. The amounts of all other ingredients of the paste wereidentical. The viscosity was 3000 cps.

The ceramic paste was coated on a glass substrate to a thickness of 110μm by a blade coater. The coating area was a 950×540 mm rectangle thatcorresponded to the lattice pattern area of the mold. Then the flexiblemold was applied along the first direction onto the 110 micron thicklayer of paste by using a 100 kg, 200 mm diameter roller at a rate of 20mm/s. Since no additional loading was given to the mold, the totalloading to the mold is 100 kg/950 mm, or about 0.105 kg/mm. Afterwards,the assembly was exposed to radiation of wavelength 400-500 nm for 30 sto cure the paste. The flexible mold was peeled from the substrate inthe first direction.

The amount of paste overflow resulting from the application step wasobtained by measuring the difference between the paste coating areabefore the application of the flexible mold and the paste coating areaafter the application of the flexible mold. The specimen of ComparativeExample 4 showed a difference in paste coating area before and after theapplication of the flexible mold of more than 50 mm in the firstdirection, which indicates that the conditions of Comparative Example 4can be said to be “paste overflow” conditions.

After the removal of the mold, the assembly of substrate and latticepattern ribs was sintered at 550° C. for 1 h to burn out the organicpart of the ribs.

After the sintering, rib defects were measured by optical microscopy.More than 100 defects were observed in the entire area (950×540 mm) ofthe specimen Comparative Example 4. Small air bubbles were also observedin this specimen.

All references and publications cited herein are expressly incorporatedherein by reference in their entirety into this disclosure. Illustrativeembodiments of this invention are discussed and reference has been madeto possible variations within the scope of this invention. These andother variations and modifications in the invention will be apparent tothose skilled in the art without departing from the scope of theinvention, and it should be understood that this invention is notlimited to the illustrative embodiments set forth herein. Accordingly,the invention is to be limited only by the claims provided below.

1. A flexible mold suitable for molding a lattice pattern, wherein thelattice pattern comprises a first set of ribs aligned in the firstdirection and a second set of ribs aligned in a second directionsubstantially orthogonal to the first direction, wherein the first setof ribs comprises a pitch of less than 500 μm, the ribs of each set havean average width, and the average width of the second set of ribs to theaverage width of the first set of ribs has a ratio of at least 1.5. 2.The flexible mold of claim 1, wherein the pitch of the first set of ribsis less than 300 μm.
 3. The flexible mold of claim 1 wherein the firstand second set of ribs have an average width ranging from 20 μm to 50μm.
 4. The flexible mold of claim 1 wherein the first and second set ofribs have heights of about 120 μm to 140 μm.
 5. The flexible mold ofclaim 1 wherein the first and second set of ribs have widths of about 20μm to 75 μm.
 6. The flexible mold of claim 1 wherein the mold is atransparent plastic mold.
 7. The flexible mold of claim 1 wherein themold is a flexible polymer sheet having a smooth surface and an opposingmicrostructured surface.
 8. A plasma display panel comprising barrierribs having a lattice pattern, wherein the lattice pattern comprises afirst set of ribs aligned in the first direction and a second set ofribs aligned in a second direction substantially orthogonal to the firstdirection, wherein the first set of ribs comprises a pitch of less than500 μm, the ribs of each set have an average width, and the averagewidth of the second set of ribs to the average width of the first set ofribs has a ratio of at least 1.5.
 9. The plasma display panel of claim8, wherein the pitch of the first set of ribs is less than 300 μm. 10.The plasma display panel of claim 8 wherein the first and second set ofribs have an average width ranging from 20 μm to 50 μm.
 11. The plasmadisplay panel of claim 8 wherein the first and second set of ribs haveheights of about 120 μm to 140 μm.
 12. The plasma display panel of claim8 wherein the first and second set of ribs have widths of about 20 μm to75 μm.
 13. The plasma display panel of claim 8, wherein a plurality ofribs of the first set of ribs are connected by intervening land regions,and further wherein the intervening land regions comprise asubstantially uniform center thickness.
 14. The plasma display panel ofclaim 8, wherein a plurality of ribs of the second set of ribs areconnected by intervening land regions, and further wherein theintervening land regions comprise a substantially uniform centerthickness.
 15. The plasma display panel of claim 8, wherein the ribscomprises a cured and fired ceramic material.